Which Way North, Part IV - The Military Lensatic Compass

I'm going to kick off our formal evaluation of compass accuracy with the a design that has been in continuous use  for over 60 years and has seen use by millions of individuals.  It is perhaps the most tested compass design in history, with documented use in jungle, desert, woodland and arctic environments around the world.  It is tried and true and is one of my favorite compass designs.

It is the US Army's Model 1950 lensatic compass.

The M1950 compass is a design born of war.  It's predecessor, the M1938 lensatic compass, was developed and adopted just as WWII opened.  It was a good design that was easy to manufacture.  Equally important, the adoption of the M1938 compass allowed the Army to standardize land navigation training, simplify it and teach it to the millions of young men who were being drafted into the Army and Marines. The experiences of war taught the Army a few things about compass design.  First, it proved that the lensatic compass design was a good one.  It was accurate, reliable and versatile.  With its compass card graduated in both degrees and mils it was usable by the both infantry and artillery.  The military liked the basic design and stuck with it.

A Model 1938 (M1938) lensatic compass manufactured
 by the Superior Magneto Company of New York
Superior Magneto appears to have been the prime supplier of this
compass design during WWII

However, wartime experience also highlighted some shortcomings in the M1938 design.  It was somewhat fragile.  While not a toy, the M1938 was lightly built - just two stamped aluminum cups fitted together to form the compass bowl and lid.  It had no mechanism to lift the compass card off of the the pivot needle when the compass was closed.  A lot of compasses were damaged when the the tip of the pivot needle gouged or cracked the pivot jewel through rough handling.  But perhaps the biggest shortcoming of the M1938 compass is that it had no dampening mechanism. This meant that the compass card would swing wildly and would take a good number of seconds to settle down to the point where the Soldier could get an accurate reading.  There were some versions of the M1938, those manufactured by the W. E. Gurley Company (a leading manufacturer of surveying instruments), that included a needle lift mechanism that with practice could be used to brake or slow the compass card oscillations.  However, the vast majority of compasses were manufactured by the Superior Magneto Corporation and did not include this needle lift mechanism.

After WWII the Army incorporated induction dampening into the M1938 design.  Induction dampening is a beautifully simple concept.  It takes advantage of the inductive magnetic field generated between a swinging compass needle and a highly conductive but non-magnetic alloy like copper.  When a magnetic needle (or bar) is placed inside a cup made of copper and the needle swings (oscillates) that movement causes a slight magnetic eddy current to form.  When the needle swings to the left the eddy current pulls it to the right.  When it swings to the right the eddy current pulls it to the left.  The eddy current is self-canceling; as the needle oscillations decrease the eddy current strength decreases and very quickly the needle settles down and is aligned with magnetic north.  Simple, elegant and effective.

A late model M1938 compass with induction dampening.  This is
a transitional design, bridging the gap between the original M1938 compass
and the the M1950.  The white colored compass bowl is actually
a copper cup that forms part of the induction dampening system.
This compass was made in December 1950 by the
Marine Compass Company out of Pembroke, Massachusetts.

However, by the late 1940s the Army decided it was time for a whole new design.  The Army took the best functional elements of the M1938 compass - the lensatic sighting design and the combined degree and mil scales on the compass card - added induction dampening, a needle lift device, a much larger sighting lens and a larger thumb loop and placed it all in a beefed-up waterproof aluminum housing.  The resulting compass was designated the M1950 Lensatic Compass.  It is a rugged, versatile device that has remained in use with the US military for over 60 years, pretty much as originally designed.

M1950 Lensatic Compass
This particular compass was manufactured in February1953 by the
Marine Compass Company out of Pembroke, Massachusetts.   The
cloudy dial cover is the result of the plastic aging.  Remember, this compass
is almost 60 years old!

Same compass with the cover closed, showing the manufacturer and
manufacturing date.

Fast forward almost 60 years and the same compass design is still in use by the US military, and it doesn't look like they have plans to switch designs any time soon.

M1950 Lensatic Compass manufactured in 2010 by the Cammenga Corporation
out of Michigan.  This is a military issue compass that uses tritium inserts
for night time illumination.

The same compass with the cover closed.  At the time of this writing
Cammenga has been the sole supplier of lensatic compasses to
the US military for over 10 years.

Since 1950 this compass has been produced by a number of manufacturers, including the Marine Compass Company, Jay-Bee, Union Instrument, Cammenga and even Lionel (yes, the train people!).  However, it seems that the single biggest manufacturer of M1950 compasses was Stocker & Yale out of Massachusetts.  I don't have any specific production numbers for these compass manufacturers so my claim is based solely on personal observation.  Based on the compasses I was issued in the Army and what I see for sale on auction sites or in surplus stores it appears that Stocker & Yale had the highest production numbers.

Now, the US military doesn't just turn to a manufacturer and say "Make it!"  Like all things military there are clearly defined specifications.  It doesn't matter if you are building an aircraft carrier or a handheld compass, there must be clearly spelled out specifications!  So it is with the M1950 Lensatic Compass.  Today's compasses are built and tested in accordance with the DOD military performance specification known as MIL-PRF-10436N (Performance Specification, Compass, Magnetic, Unmounted, Lensatic, Luminous, 5 Degree and 20 Mil Graduations, With Carrying Case).

MIL-PRF_10436N
The document that spells out the design, construction,
performance and testing requirements for the
lensatic compass

(I should note here that the current specification does not use the 'M1950' designation.  I'm not sure when the US military dropped the designation, but for our purposes we'll continue to call it the M1950.  It's the same compass.)

The discussion of this performance document is important because the M1950 compass is the only US produced handheld compass I am aware of that is built to a specific performance specification, and is regularly evaluated against this performance specification by an outside agency.  If any test batch of compasses fails the evaluation the devices never make it out of the factory.  The M1950 is a purpose-built device designed to meet a clearly laid out specification not just for accuracy but for shock resistance, water resistance, illumination, thermal shock, durability and service life.  Manufacturers of other compasses may have their own internal standards (and many are quite good), but the M1950 is the only handheld compass you can buy that is designed to meet demanding military standards and is rigorously tested by an independent agency to ensure it meets those standards.

So just how good is the M1950 compass in the real world?  Pretty damned good!  The 58-year old example I show above that was made by the Marine Compass Co. is still perfectly serviceable and would probably meet all of today's performance specifications for accuracy and durability.  I have other examples in my collection that have clearly seen hard use, some with broken components or cracked dials, but they still provide reliable and accurate readings. The M1950 compass is a device that is hard to kill.

I believe that the key to the M1950's ruggedness is the fact that is it not a liquid dampened design.  Liquid dampening (where the compass needle or card is suspended in fluid to reduce oscillation) is very effective but is a more fragile design than the induction dampening used in the M1950.  With the liquid filled design the compass needle or card must be sealed inside a leak proof capsule*.  The problem is, compass manufacturers have not yet figured out how to make a leak proof capsule.  I have over 15 liquid filled compasses in my personal collection.  About half have air bubbles inside the capsule, a sure indicator that the fluid is leaking.

How accurate is the M1950 compass?  Every M1950 I've used (and after a 23 year Army career I've used a lot of them) has at least met, and many exceeded, the performance specification for accuracy.  Now this is where I need to come clean on my evaluation of the M1950.  It is not the most precise handheld compass available.  This compass' biggest design limitation is that the compass card is divided into only 5 degree increments; pretty coarse even for handheld use.

Compass card of the M1950 compass.
Note the inner degree ring (in red) laid out in 5 degree increments.  The outer
ring (in black) is set out on mils (6400 mils to a circle).  

This means that the average user, the common Soldier, can only discern and measure to half of that increment - 2.5 degrees.  Experienced users - mostly infantry and artillery Soldiers who use a compass regularly - can frequently get accurate readings to between 1.5 - 2.0 degrees. But to be realistic, 2.5 degrees is about as good as anyone can expect to get with this compass card layout.  The M1950 compass card design is a compromise.  The military needed to include a mils scale for use by the Field Artillery.  Mils offer more discreet division of the circle (a mil is 1/6400th of a circle), allowing for more precise azimuth determination - very important when you are calling in artillery strikes on distant targets.  To accommodate the mils scale it needed to be printed at the outer edge of the compass card, leaving less space to print the degrees scale.  This results in a coarse, less precise degree scale.

The military performance specification states that the compass must be accurate to within 40 mils.

"4.4.1.8 Magnetic performance and compass error.  The compass shall be placed in a horizontal position on a fixed point and by means of the sighting mechanism, the compass shall be sighted on three targets of known magnetic azimuths approximately 120 degrees apart.  With no remedial action by the operator, before, at, or after, a reading shall be taken at each target.  The difference between the known azimuths and readings taken is the compass error.  An error greater than 40 mils or failure of the compass to function correctly shall constitute failure of this test."

Since one degree = 17.8 mils, 40 mils is slightly less than 2.5 degrees.  Let's round up and call it 2.5.

I have tested my 2010 production Cammenga compass at a known azimuth station and found it to be accurate to just over 2 degrees when used in the handheld mode and sighting on targets up to 150 feet away.  This compass very easily meets the performance specification.

Before I wrap up this blog post I need to add that the M1950 compass was merely one component of a land navigation system that the Army developed and adopted at the end of WWII.  Along with the M1950 compass came dramatic changes in how the Army mapped the world, developing standardized maps with overprinted grids (the Military Grid Reference System) and plotting tools.  It was all designed to simplify land navigation for the common Soldier, and is was so successful that the methodology is still in use today.

The US Army's standardized land navigation 'system' included the M1950 compass,
standardized topographic maps, plotting tools and training materials.  It was an
extraordinarily successful program that is still used today.

Let's wrap this up.  Here is my bottom line - I consider the the M1950 compass to be the best general purpose handheld compass available.  It is a proven design that is built and tested to exacting standards.  They are readily available new or used to civilians and are one of the best examples of trickle-down military technology I've seen.  If you spend any time in the outdoors you need a compass.  You might as well get the best available.  Get a M1950 Lensatic Compass.

Thanks!

Brian

*One compass manufacturer, K&R out of Germany, claims to make a leak proof liquid capsule but I don't think they have been on the market long enough to have proven the claim.